An adaptive array radio base station employing an array antenna is recently put into practice as a radio base station for a mobile communication system such as a mobile telephone. The operation principles of such adaptive array radio base stations are described in the following literature, for example:
B. Widrow, et al. “Adaptive Antenna Systems,” Proc. IEEE, vol. 55, No. 12, pp. 2143–2159 (December 1967).
S. P. Applebaum, “Adaptive Arrays,” IEEE Trans. Antennas & Propag., vol. AP-24, No. 5, pp. 585–598 (September 1976).
O. L. Frost, III, “Adaptive Least Squares Optimization Subject to Linear Equality Constraints,” SEL-70-055, Technical Report No. 6796-2, Information System Lab., Stanford Univ. (August 1970).
B. Widrow and S. D. Stearns, “Adaptive Signal Processing,” Prentice-Hall, Englewood Cliffs (1985).
R. A. Monzingo and T. W. Miller, “Introduction to Adaptive Arrays,” John Wiley & Sons, New York (1980).
J. E. Hudson, “Adaptive Array Principles,” Peter Peregrinus Ltd., London (1981).
R. T. Compton, Jr., “Adaptive Antennas-Concepts and Performance,” Prentice-Hall, Englewood Cliffs (1988).
E. Nicolau and D. Zaharia, “Adaptive Arrays,” Elsevier, Amsterdam (1989).
FIG. 17 is a model diagram schematically showing the operation principle of such adaptive array radio base stations. Referring to FIG. 17, an adaptive array radio base station 1 includes an array antenna 2 formed by n antennas #1, #2, #3, . . . , #n. A first area 3 with slant lines shows the range capable of receiving radio waves from the radio base station 1. A second area 7 with slant lines shows the range capable of receiving radio waves from another radio base station 6 adjacent to the radio base station 1.
In the area 3, a mobile telephone 4 serving as a terminal of a user A transmits/receives a radio signal to/from the adaptive array radio base station 1 (arrow 5). In the area 5, on the other hand, a mobile telephone 8 serving as a terminal of another user B transmits/receives a radio signal to/from the radio base station 6 (arrow 9).
If the radio signal employed in the mobile telephone 4 of the user A is by chance equal in frequency to that employed in the mobile telephone 8 of the user B, the radio signal from the mobile telephone 8 of the user B may act as an undesired interference signal in the area 3 depending on the position of the user B, to be mixed into the radio signal between the mobile telephone 4 of the user A and the adaptive array radio base station 1.
In this case, the adaptive array radio base station 1 receives the radio signals from the users A and B in a mixed state if taking no measures, to disadvantageously disturb communication with the user A.
In order to eliminate the signal from the user B from the received signals, the adaptive array radio base station 1 employs the following structure and processing.
[Structure of Adaptive Array Antenna]
FIG. 18 is a block diagram showing the structure of an adaptive array 100. Referring to FIG. 18, the adaptive array 100 is provided with n input ports 20-1 to 20-n, in order to extract a signal of a desired user from input signals including a plurality of user signals. Signals received in the input ports 20-1 to 20-n are supplied to a weight vector control part 11 and multipliers 12-1 to 12-n through switching circuits 1-1 to 10-n.
The weight vector control part 11 calculates weight vectors w1i to w1n with a training signal corresponding to the signal of a specific user previously stored in a memory 14 and an output of an adder 13. Each subscript i indicates that the weight vector is employed for transmission/receiving to/from an i-th user.
The multipliers 12-1 to 12-n multiply the input signals from the input ports 20-1 to 20-n by the weight vectors w1i to w1n respectively and supply the results to the adder 13. The adder 13 adds up the output signals from the multipliers 12-1 to 12-n and outputs the result as a received signal SRX(t), which in turn is also supplied to the weight vector control part 11.
The adaptive array 100 further includes multipliers 15-1 to 15-n receiving an output signal STX(t) from the adaptive array radio base station 1, multiplying the same by the weight vectors w1i to w1n supplied from the weight vector control part 11 and outputting the results. The outputs of the multipliers 15-1 to 15-n are supplied to the switching circuits 10-1 to 10-n respectively. The switching circuits 10-1 to 10-n supply the signals received from the input ports 20-1 to 20-n to a signal receiving part 1R in receiving, while supplying signals from a signal transmission part 1T to the input/output ports 20-1 to 20-n in signal transmission.
[Operation Principle of Adaptive Array]
The operation principle of the signal receiving part 1R shown in FIG. 18 is now briefly described.
In order to simplify the illustration, it is hereafter assumed that the number of antenna elements is four and the number of users PS from which signals are simultaneously received is two. In this case, signals RX1(t) to RX4(t) supplied from the antennas to the receiving part 1R are expressed as follows:RX1(t)=h11Srx1(t)+h12Srx2(t)+n1(t)  (1)RX2(t)=h21Srx1(t)+h22Srx2(t)+n2(t)  (2)RX3(t)=h31Srx1(t)+h32Srx2(t)+n3(t)  (3)RX4(t)=h41Srx1(t)+h42Srx2(t)+n4(t)  (4)where RXj(t) represents a signal received in a j-th (j=1, 2, 3, 4) antenna, and Srxi(t) represents a signal transmitted from an i-th (i=1, 2) user.
Further, hji represents a complex factor of the signal from the i-th user received by the j-th antenna, and nj(t) represents noise included in the j-th received signal.
The above equations (1) to (4) are expressed in vector forms as follows:X(t)=H1Srx1(t)+H2Srx2(t)+N(t)  (5)X(t)=[RX1(t),RX2(t), . . . ,RXn(t)]T  (6)Hi=[h1i,h2i, . . . ,hni]T, (i=1,2)  (7)N(t)=[n1(t),n2(t), . . . ,nn(t)]T  (8)
In the above equations (6) to (8), [ . . . ]T shows transposition of [ . . . ]. Here, X(t) represents an input signal vector, Hi represents a received signal factor vector of the i-th user, and N(t) represents a noise vector respectively.
As shown in FIG. 18, the adaptive array outputs a signal formed by multiplying the input signals from the respective antennas by the weighting factors w1i to w1n as the received signal SRX(t). The number n of the antennas is four.
When extracting the signal Srx1(t) transmitted from the first user, for example, the adaptive array operates under the aforementioned preparation as follows:
An output signal y1(t) from the adaptive array 100 can be expressed by multiplying the input signal vector X(t) by a weight vector W1 as follows:y1(t)=X(t)W1T  (9)W1=[w11,w21,w31,w41]T  (10)
The weight vector W1 has the weighting factor wj1 (j=1, 2, 3, 4) multiplied by the j-th input signal RXj(t) as its element.
Substitution of the input signal vector X(t) expressed in the equation (5) into y1(t) expressed in the equation (9) gives the following equation:y1(t)=H1W1TSrx1(t)+H2W1TSrx2(t)+N(t)W1T  (11)
When the adaptive array 100 ideally operates, the weight vector control part 11 sequentially controls the weight vector W1 by the well-known method described in the above literature, to satisfy the following simultaneous equations:H1W1T=1  (12)H2W1T=0  (13)
When the weight vector W1 is completely controlled to satisfy the equations (12) and (13), the output signal y1(t) from the adaptive array 100 is ultimately expressed as follows:y1(t)=Srx1(t)+N1(t)  (14)N1(t)=n1(t)w11+n2(t)w21+n3(t)w31+n4(t)w41  (15)In other words, the signal Srx1(t) transmitted from the first one of the two users is obtained as the output signal y1(t).
Referring to FIG. 18, the input signal STX(t) for the adaptive array 100 is supplied to the transmission part 1T in the adaptive array 100 and supplied to first inputs of the multipliers 15-1, 15-2, 15-3, . . . , 15-n. The weight vectors w1i, w2i, w3i, . . . , wni calculated by the weight vector control part 11 on the basis of the received signals in the aforementioned manner are copied and applied to second inputs of the multipliers 15-1, 15-2, 15-3, . . . , 15-n respectively.
The input signal weighted by the multipliers is transmitted to the corresponding antennas #1, #2, #3, . . . , #n through the corresponding switching circuits 10-1, 10-2, 10-3, . . . , 10-n respectively, and transmitted into the area 3 shown in FIG. 17.
The users A and B are identified as follows: The radio signal from each mobile telephone is transmitted in a frame structure. The radio signal from the portable telephone is roughly formed by a preamble formed by a signal series known to the radio base station and data (voice etc.) formed by a signal series known to the radio base station.
The signal series of the preamble includes a signal string of information for determining whether or not the user is a desired user for making communication with the radio base station. The weight vector control part 11 of the adaptive array radio base station 1 contrasts the training signal corresponding to the user A fetched from the memory 14 with the received signal series and performs weight vector control (decision of the weighting factor) to extract a signal seeming to include the signal series corresponding to the user A.
FIG. 19 is a diagram imaging transfer of the radio signal between the user A and the adaptive array radio base station 1.
The signal transmitted through the same array antenna 2 as that in receiving is subjected to weighting targeting the user A similarly to the received signal, and hence the transmitted radio signal is received by the mobile telephone 4 of the user A as if having directivity to the user A.
When outputting the radio signal to the area 3 showing the range capable of receiving radio waves from the adaptive array radio base station 1 as shown in FIG. 17 while properly controlling the adaptive array antenna as shown in FIG. 19, it follows that the adaptive array radio base station 1 outputs a radio signal having directivity targeting the mobile telephone 4 of the user A as shown in an area 3a in FIG. 19.
As described above, the adaptive array radio base station 1 can transmit/receive a radio signal having directivity targeting a specific user, whereby a path division multiple access (PDMA) mobile communication system can be implemented as described below.
In order to efficiently utilize frequencies in a mobile communication system such as a mobile telephone, there are proposed various transmission channel allocation systems including the aforementioned PDMA system.
FIG. 20 shows arrangements of channels in various communication systems including frequency division multiple access (FDMA), time division multiple access (TDMA) and PDMA systems.
With reference to FIG. 20, the FDMA, TDMA and PDMA systems are now briefly described. In the FDMA channel allocation system shown in FIG. 20(a), analog signals from users 1 to 4 are frequency-divided and transmitted through radio waves of different frequencies f1 to f4. The signals from the users 1 to 4 are separated through a frequency filter.
In the TDMA system shown in FIG. 20(b), a digitized signal from each user is time-divided every constant time (time slot) and transmitted through radio waves of different frequencies f1 to f4. The signal from each user is separated through a frequency filter and time synchronization from a base station and a mobile terminal unit of each user.
On the other hand, the PDMA system shown in FIG. 20(c) spatially divides a single time slot at the same frequency for transmitting data of a plurality of users. In the PDMA system, the signal of each user is separated through a frequency filter, time synchronization between a base station and a mobile terminal unit of each user and a mutual interference eliminator employing an adaptive array or the like.
When employing the PDMA system, as shown in FIG. 19, not only radio signals transferred between different radio base stations and two users corresponding to the radio base stations must be separated not to mutually interfere with each other but also mutual interference between radio signals transmitted/received to/in different users with the same frequency and the same time slot in the area belonging to the same adaptive array radio base station 1 must be eliminated.
In the example shown in FIG. 19, it is possible to prevent the radio signal from the terminal of the user B transmitting/receiving the radio signal to/from the adjacent base station from interfering the radio signal of the user A transmitting/receiving the radio signal to/from the adaptive array radio base station 1 by utilizing directivity through the adaptive array antenna 2.
There may be such a situation that condition of radio transmission path between a desired user A and adaptive array radio base station 1 changes because of rapid movement of the user A, resulting in a change in intensity of the radiowave signals (fading).
In a communication utilizing space division multiplexing such as in the PDMA system, when the degree of such fading increases, it becomes difficult at adaptive array radio base station 1 to control directivity to the desired user A, and directivity would be deviated from the user A.
The present invention was made to solve the above described problem, and an object is to provide, in a system of transmitting/receiving radiowave signals in accordance with the PDMA system, a radio apparatus having transmission directivity, a method and a control program of controlling transmission directivity that prevent deviation of directivity to a desired terminal caused by fading.
Another object of the present invention is to provide, in a system for transmitting/receiving radiowave signals in accordance with the PDMA system, a radio apparatus having transmission directivity, a method and a control program of controlling transmission directivity that enable more exact directivity control to a desired terminal, by utilizing, in addition to transmission directivity control based on fading, transmission directivity control based on received power.